Telephone system



Dec. 17, 1940# H. HOVLAND TELEPHQNE SYSTEM Filed Feb. l, 1938 2Sheets-Sheet l ATTORNEY Dec. 17, 1940. H. HovLAND TELEPHONE SYSTEM FiledFeb. 1, 1938 j sheets-sheet 2 N w aww. m. w

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Patented Dec. 17, 1940 UNITED y STATES TELEPHONE SYSTEM Henry Hovland,Williston Park, N. Y., assigner to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationFebruary 1, 1938, Serial No. 188,053

11 Claims.

-This invention relates to telephone systems and particularly to systemsin which automatic Aswitches are employed in establishing telephoneconnections.

It is an object of the invention to enable a Vmore eicient use ofinteroiice trunks in automatic and semi-automatic telephone systems.Other objects are the prevention of double connections and theprevention of false charging on calls from 'message rate .and prepaymentcoin lines in systems comprising two-way interofce trunks.

The invention is an improved two-way trunk circuit'which is arranged toprevent the establishment of a talking connection if the trunk is seizedat both ends and to allow completion of a call over the trunk in casethe trunk is seized at one end on a second call before the connectionhas been released at the other end on a preceding call. According to afeature of the invention, a connection over a two-way trunk arranged forcomposite signaling thereover is controlled by the calling party and thetrunk isheld -busy at the outgoing end, after release'by a callingparty, for a longer interval of time than it is 'held busy at theincoming end so as to prevent seizure of the trunk on another callvbefore the switches at the incoming end of the trunk on the iirst callhave returned to normal. Means are provided for transmitting a tonesignal to the calling subscriber when the subscribers line is connectedto a two-Way trunk which has also been seized at the distant end. Afurther feature is a two-way trunk circuit arranged to momentarilydisconnect the guarding potential from the test conductor at theoutgoing end after release of the trunk on a firstv call to enablerelease of the train of switches at that end in case the trunk is againseized at the distant end von a second call before this train ofswitches has been released.

To facilitate a clear and complete description of the invention, .anautomatic telephone system arrangedV in accordance with the invention isrepresented schematically in the drawings which form a part of thisspecification. The invention is, however, not limited to the speciiicarrangements shown in the drawings and is generally applicable to .allautomatic telephone systems comprising two-way trunks.

Referring to the drawings, which consist of two gures:

Fig. 1 shows subscribers lines LI and L3, aV

line-finder switch LFI and an associated selector switch SI, a two-waytrunk circuit TI and an associated incoming selector switch ISI, and

a connector switch CI, Iall of which are in one cnice of a telephonesystem; and

Fig. 2 shows subscribers lines L2 and L4, a `line-finder switch LF2 andan associated selector switch S2, a two-way trunk circuit T2 and `s anassociated incoming selector switch IS2, and a connector switch C2, allof which are in another oice of the system.

The telephone stations A, B, C and D are of the usual type employed incommon battery 10 telephone systems and each includes a dial orequivalent impulse sender for use in controlling the operation of theselector and connector switches by which desired connections areestablished. Although lines LI and L2 are shown 15 connected only toterminals in the banks `oi `line-finder switches'and lines L3 and L4`are shown connected only 'to terminals in the banks of connectorswitches, all of these lines are connected to terminals in the banks ofboth line- 20 nder and connector switches so that calls can beoriginated and received at each of the subscribers stations. i

The line-finder, selector and connector switches are ofthe well-knowntwo-motion stepby-step type. For a description of the structure of suchswitches and their operation when used as selectors and connectors,reference may be had to pages 53 to 67 of the second edition ofAutomatic Telephony by Smith and Campbell.

For a description of the operation of such linender switches, referencemay be had to Patent 1,799,654, granted to R. L. Stokely, April 7,41931. Each of the switches LFI, LF2, SI, S2, ISI, IS2,

CI and C2 is represented by a set of brushes and 35 a single set ofterminal, all magnets, relays and other associated apparatus beingomitted with the exception of the line and release relays of theincoming selectors ISI and IS2. Although not shown, additional selectorswitches may be `included in the train required for completion of adesired connection.

The like trunk circuits TI and T2 are shown in detail and includerepeating coils RCI 'and RC2, and composite sets terminating the ends ofthe interoflice trunk T. The corresponding relays, condensers,resistors, retard coils and conductors of trunk circuits TI and T2 aregiven reference characters having like tens and units digits. Thecomposite set at one end of trunk T comprises condensers |53, |54, |59and |60, retard coils |55, |56, |51 and |58 and the composite signalingrelay |50. The composite set at the other end of the trunk compriseslike elements with corresponding reference characters. A

ground potential compensating circuit is traced from ground throughresistor |5I and the upper winding of relay |50, lower windings ofretard coils |55 .and |56, lower conductor of trunk T,

- relay 250 being normally connected through back contacts of relays2I|, 208 and 2I0 to ground. The lower winding of each of relays |50 and250 is connected in series with a biasing battery to hold the armaturesof these relays in normal position.

Assume iirst that trunk T is used on a call from station A to station D.Upon removal of the receiver from the receiver hook at station A, a linender LFI operates in well-known manner to eX- -tend the line LI to afirst selector Sl. In response to the dialing of the rst digit of thecalled subscribers number, the selector SI selects the desired group andan idle set of terminals therein, the line LI being thereby connected toan idle two-way trunk circuit T| Relay ||0 is operated by the current ina circuit from battery through the upper winding of relay I I0, innerupper back contact of relay |02, left upper winding of repeating coilRCI, outer upper back contact of relay |02, upper back contact of relayIOI, upper brushes of selector SI and line finder LFI, through the lineconductors and telephone instrument at station A, middle brushes of linefinder LFI and selector SI, lower back contact of relay |0I, outer lowerback contact of relay |02, lower left winding of repeating coil RCI,inner lower back contact of relay |02, and lower winding of relay I|0 toground. Relay IIO disconnects the ground normally connected to thejunction between the lowermost and middle windings of signaling relay|50 and closes circuits from signaling battery through resistance lamp|09, upper frontcontact of relay I I 0, back contacts of relays |08 andII I, thence through resistor |52 to ground, through the lowermostwinding of relay |50 to biasing battery, and through the middle windingof relay |50, upper windings of retard coils |55 and |56, upperconductor of trunk T, upper windings of retard'coils 256 and 55, ymiddlewinding of relay 250, back contacts of relays 2| I, 208 and '2|0 toground. Relay |50 does not operate since the current through thelowermost winding of relay |50 is electromagnetically opposed to andneutralizes the current through the middle winding; but signaling relay250 at the other end of trunk T is operated, the energization of ,themiddle winding of relay 250 being strong enough to overcome theenergization of the biasing winding. The operation of relay I I0 alsocloses a circuit frombattery through the winding of slow-to-releaserelay |28, lower front contact of' relay ||0, to ground at the uppermostback contact of relay |20. At its inner upper front contact, relay |28connects ground through sleeve conductor |00 to the sleeve terminals oftrunk TI in the banks of selector switches having access thereto toguard against seizure of trunk circuit TI by another selector. At itsinner lower front contact, relay |28 lcloses a` circuit which includes aback Contact of `relay I||`for operating the slow-to-release relayIBI..Relay |.3I disconnects ground from conductor |35 leading to the grouptraffic register circuit, closes a circuit for operating theslow-to-release relay |29 and bridges the condenser I I8 across theinner ends of the left windings of repeating coil RCI. Relay |29connects ground through its outer upper front Contact and a back Contactof relay III to the sleeve conductor |00 and closes a circuit throughits outer lower front contact for operating the slow-to-operate relay|30.

The aforementioned operation of relay 250 of trunk circuit T2 closes acircuit for operating relay 232. Relay 232 closes a circuit foroperating relay 22|, closes a circuit for energizing the lower,polarizing winding of relay 208, closes a circuit for energizing theupper winding of relay 223, and closes a circuit for charging cond-enser233 in series with resistor 234 and the winding of relay 220. Theenergization of the upper winding of relay 223 is eiective to close andhold closed its left contact. Relay 220 operates while condenser 233 ischarging and then releases. Relay 221 locks through the outer lower backContact of relay 206, conductor 236 and both upper front contacts ofrelay 221; disconnects ground from conductor 235 leading to the grouptraffic register circuit; closes a circuit foroperating relay 202;closes a circuit through both of its `upper front contacts and conductor236 for energizing the lower polarizing winding of relay 208, closes acircuit through both of its upper front contacts, conductor 236 and theleft contact Yof relay 223 for operating relay 206; and connects groundthrough its inner upper front contact, conductor 231, the outer upperback contact of relay 2II, and through conductor 200 to sleeve terminalsin the banks of selector S2 and other selectors to guard against seizureof trunk circuit TI by any of these selectors. The operation of relay206 closes a circuit for energizing the lower winding of polarized relay223, and closes a circuit for operating the line relay 213| of incomingselector IS2. The i energization of the lower winding of relay 223opposes'and overcomes the energization of its upper Winding to actuateits armature to close the right contact. Relay 223 opens the operatingcircuit of .relay 206 but relay 206 remains operated in a lockingcircuit under the control of relays 220 and 227. The aforementionedcircuit for operating relay 24| is traced from ground through its rightwinding, conductor 239, lowermost front contact of relay 202, lowerright winding of repeating coil RC2, inner lower front Contact of relay202, upper winding of relay 208, upper front contact of relay 205,resistor 203, inner upper front contact of relay 202, upper rightwinding of repeating coil RC2, outer upper front contact of relay 202,conductor 238, and through the left winding of relay 24| to battery.Relay 24| closes a circuit for operating relay 242.

When the subscriber at station A dials a digit of the number of thecalled station, relay ||0 releases and reoperates in response to eachdial impulse, thereby momentarily transferring the junction between thelower and middle windings of relay |50 from battery to ground to causethe release and reoperation of relay 250 of trunk circuit T2. Therelease of relay I0, in response to the iirst impulse resulting from thedialing of a digit, closes a circuit which includes the outer upperfront contact of relay ISI, for operating the slow-to-release relay |I|.Relay sh0rtcircuits the left windings of repeating coil RCI ,until allof the impulses inthe train have been of relay 221.

received by relay I l0; and also short-circuits the upper winding ofrelay |08 but without useful function on a call outgoing from trunk cir-'cuit TI. Being slow-to-release, relay |28 remains operated while relayI I0 is responding to dial impulses. The release and reoperation ofVrelay 250 of trunk circuit T2`in response to each dial impulse receivedby relay ||0 of trunk circuit TI, causes the release and reoperation of`relay 232. Each release of relay 232 causes disof relay 223. The lowerwinding of relay 223 continues to be energizedwhile condenser 224 ischarging; butwhen the charging current has decreased sufficiently, theupper Winding of relay 223 actuates the armature to open its rightcontact and close its left contact. Relay 220 having released after itsmomentary operation, the closing of the'left contact of relay 223 causesthe reoperation of relay 206 and relay 206 again `closes the impulsecircuit and the circuit for energizing the lower winding of relay 223 sothat relay 233 again closes its right contact. The reoperation of relay206 also causes the discharge of condenser 224. The capacity ofcondenser 224 and the resistances of resistors 222 and 225 are of suchvalues as to delay the release of relay 223 a definite and predeterminedinterval of time, so that relay 206 is released and holds open thecircuit of relay 24| for an interval sufficient to insure release ofrelay 24| and actuation of the vertical magnet of selector ISZ. Uponrelease of relay 232 responsive to the rst release of relay 250, relay226 is operated by its upper winding in series with the winding of relay221. Relay 226 opens its operating circuit but locks through its lowerwinding under control of relay 223 and closes a circuit for operatingrelay 2|1; this circuit includes the upper front contacts of relay 221,upper front contact of` relay 226, and the inner lower front contact Assoon as relay 223 opens its right contact, relay 226 releases and opensthe circuit of relay 2I1; but relay 2|1 is slow in releasing and remainsoperated until relay 226 releases after the last impulse in the trainhas been received. Each impulse of the train causes the above cycle ofoperations of relays 232, 220, 206, 223 and 226 to be repeated. Relay2|1 short-circuits the right windings o-f repeating coil RC2 and theupper winding of relay 208to reduce the impedance in the impulse circuitwhile each train of impulses incoming toV relay 250 is being repeated bythe opening and closing of the upper contacts of relay 206. Relay 221 isslow to release and since'its locking `circuit is closed each time relay206 releases and since its winding is momentarily held energized invseries with the upper winding of` relay 226 each time relay 206operates, relay 221 does not release while dial impulses are beingreceived by relay 250 and repeated by relay 206.

Relay 24| of incoming selector' ISZ releases and reoperates in responseto each impulse of the first train repeated by the contacts of relay 206thereby causing the brushes of selector IS2 to be selectively advancedto the desired group of terminals in usual andwell-known manner. Relay242 is slow to release and remains operated idle set of terminals in theselected group and extend the impulse repeating circuit f-rom conductors238 and 239 tothe line relay (not shown) of the connector circuit C2.Relays 24| and 242 are thereupon released and sleeve conductor 231 isconnected to holding ground potential in the connector circuit in usualmanner. The last two trains of impulses repeated by relay 206 areeiective to advance the brushes of connector switch C2 to establishconnection with the called line L4; inusual and well-known manner. Theline being idle, the usual ringing circuit is closed and when thereceiver is removed at station D to answer the call, the connector C2reverses the current through conductors 238 and 239 and the upperwinding of polarized relay 208 to cause the operation of relay 206.Relay 208 disconnects ground from the lower windings of relay 250 andconnects battery thereto to cause the operation of relay |50 of trunkcircuit TI as an answering supervisory signal. Relay |50 closes acircuit, which includes the front contact of relay |30, for operatingrelay |0I; and relay |0| reverses the current through the calling lineLI.

If the called subscriber returns the receiver to normal before thecalling subscriber releases the connection, the resulting reversal ofcurrent by the connector circuit causes the -release of relays 208, |50and |0I. When the calling subscriber returns the receiver to normal,relay ||0 releases thereby reconnecting the junction between the lowerwindings of relay |50 to ground 'to cause the lrelease of relays 250 and232 of trunk circuit T2. The release of relay 232 causes the dischargeof condenser 233. Since relay 232 does not reoperate, relays 206 and 221release, causing the release of relay 202, and disconnecting ground fromconductor 231. Relay 2|1 operates when relay 206 releases and releaseswhen relay 221 releases. The release of relay 206 opens the outgoingsupervisory circuit over conductors 238 and 239 to cause the release ofthe line relay of the connector circuit and the release of relay 208 ifnot already released. Whenthe connector C2 also disconnects holdingground from sleeve conductor 231, selector IS2 is returned to normal,and trunk circuit T2 is again ready for seizure by a selector S2. Theconnector is returned to normal in usual manner as soon as the receiveris returned toinormal at the called station. The aforementioned releaseof relay |I0 also causes the operation of relay ||1, and the successiverelease of slow-to-release relays |28, I3| and |29 and the release ofslow-to-operate relay |30. The release of relay |3| 'causes the releaseof relay ||1. Relays |3| and |29 maintain the connection of ground tothe sleeve conductor |00 for an interval of time after relay I0 releasesso that trunk circuit T| cannot be again seized on another call untilthe selector IS2 has been restored to normal. When ground isdisconnected from conductor |00, the selector SI and .line finder LFIare restored to normal in usual and well-known manner.

On 'a call from station B to station C, line L2 is extended through linender switch LF'2 and by the selective operation of selector S2 undercontrol of the dial at station B to an idle trunk circuit T2. Thereupontrunk circuit T2 operates as did trunk circuit TI, and "trunk circuit T|operates as did trunk circuit T2, on the call above describedfromstation A to station D.

. 'In case thel trunkcircui-t Tl is seized byl a selector Si at aboutthe same time that trunk circuit T2 is seized by a selector- S2, relays110,

128, llfand 129 of trunk circuit TI operate as hereinfbefore describedin. about the same interval of 'time that relays 210, |50-, 132, 121 and|02 operate due to seizure of trunk circuit T2; and relays 210, 228, 231and 229 of trunk. circuit T2 operate' in about the same interval of timethat relays 11d, 2513, 232, 221 and 202 operate due to seizure of trunkcircuit T1. The operation of relay 129 closes a circuit for operatingrelay 111; andthe operation of relay 229 closes a circuit for operatingrelay 21 1. Relays ISU-and 230 are slow in operating so that relays 1 1I and 211 will operate and prevent the operation of relays 130 and 230.Relay 111 locks under control of relay 128, connects relay 110 directlyto the calling line, connects ground to the junction between the-lowerwindings of relay 150 to cause the release of relay 250, disconnectssleeve conductor 100 from the sleeve conductor 13T of incoming selectorISI, connects ground to conductor 114 to start the intcrrupter mctor11.5 if `not already operating, closes a circuit path from tone source 113 through Y condenser 112 to one of the conductors of the calling line,closes a circuit for operatingrelay 102 ifl not already operated, andcauses the release of relays 13E), 131 and129. A similar series ofoperations results from the operation of relay 211 of trunk circuit T2.Each of the calling stations is thus disconnected from the trunk so thata wrong connection does not result. When the calling subscriber at thestation connected to trunk TI returns the receiver to normal, relays111), 128, 111 and |02 release; and when the calling subscriber at thestation connected to trunk T2 returns the receiver to normal, relays210, 228, 211 and 2112 release.

vIn case the trunk circuits T1 and T2 are both seized at about the sametime, the operating times of the various relays may be such that relayof trunk circuit T1 operates before relay l 1 1 operates or that relay230 of trunk `circuitfTZ operates before relay 211 operates. In thiscase if relays |11 and 230 are operated, the calling station connectedto trunk circuit T1 is blocked and receives a tone as above described;but the call incoming to trunk circuit'TI from trunk circuit T2 will becompleted. Or if relays 21 l and |30 are operated, the calling stationconnected to'trunk circuit T2 is blocked and receives a tone but thevcall in-y coming to trunk circuit T2 from trunk circuit T1 will becompleted. When the calling subscriber whose call is blocked returns thereceiver to normal, the releasev of relay 110 or 210 causes the releaseof relay |28 or 228. 'I'herelease of relay 128 or 228 disconnects groundfrom the sleeve conductor 100 or 2130 and causes the release of relay111 or 211. Relays 11| and 211 are slow in releasing lto insure releaseof the selector S1 and line finder LFI or selector S2 and line finderLFZ before reconnecting conductor 100 or 200 to ground and to the sleeveconductor 13'1 or 231 of the associated incoming selector ISI or IS2.

From the foregoing description, it is apparent that the trunk circuit atthe called end of the trunk is restored to normal by the successiverelease of two slow-to-release relays,for instance, relay 221- of trunkcircuit'TZ and the release relay of the associated connector; whereasthe trunk circuit atthe calling end is notrestored to normal untilcompletion of the successive release of three slow-to-release relaysforinstance, relays 128, 131 and 129 oftrunk circuit T1. The trunk circuitat the called end may, therefore, be again seized on a second callbefore the trunk circuit at the calling end ofthe iirst call .hasrestored to normal. In this case, operation of the composite signalingrelay, or 250, before the near trunk circuit has been restored to normalcausesthe operation of relays 132 and 1 I 1, or relays 232 and 2H, inmultiple. Relay 111, or 211, locks under control of relay 12S, or 229,and disconnects ground from the sleeve conductor 100, or 200, to allowthe line iinder and first selector switches used on the first call torestore to normal. When relay- 129, or 229, releases, relay |11, or 211releases and the sleeve conductor 100, or 200, is then connected to thesleeve conductor of the associated incoming selector to guard againstseizure by one of the rst selectors. 'Ihe second call is thereuponcompleted by operation of the incoming selector anda connector vat theone end of the trunk under the control of the dial at the callingstation at the other end of the trunk.

What is claimed is:

l. In a telephone system, subscribers lines, a two-way trunk arrangedfor composite signaling thereover, means comprising a selector switchfor connecting a calling line to said trunk, means comprising anotherselector switch for connecting said trunk to a called line, meansresponsive to seizure of said trunk for marking said trunk busy at bothends, and means at each end of the trunk and controlled by the callingsubscriber for terminating the marking of said trunk as busy, said meansbeing effective to terminate the busy marking of said trunk at `rthe endto which the calling line is connected after the marking of said trunkas busy is terminated at the end to which the called line is connected.

2. In a telephone system, subscribers lines, a two-way trunk arrangedfor composite signaling th'ereover, means comprising a selector switchfor connecting a calling line to said trunk, means comprising anotherselector switch for connecting said trunk to a called line, meansresponsive to seizure of said trunk for marking said trunk busy at bothends, means responsive to the release of the connection by the callingsubscriber for terminating the marking of said trunk as busy and forcausing the return of said selector switches to normal, and means fordelaying the return of the switch through which the calling line isconnected to said trunk until a suiicient interval of time has Yelapsedfor said switch through which the trunk is connected to the called lineto have been restored to normal.

3. In a telephone system according to claim 2, r

means responsive to Vseizure of said trunk at the end connected to saidcalled line for temporarily interrupting the marking of the trunk asbusy at the end connected to said calling line if the last-mentionedseizure occurs prior to the release of the switch through which saidcalling line is connected tothe trunk.

4. In a telephone system, subscribers lines, a two-way trunk, meanscomprising a selector switch for connecting a calling line to saidtrunk, means comprising another selector switch for connecting saidtrunk to a called line, means responsive to seizure of said trunk formarking said trunk busy at both ends, means at each end of said trunkeffective upon simultaneous connection of calling lines to both ends ofsaid trunk for preventing the connection of said trunk to a called lineat either end, means at one end of the trunk `for transmitting to acalling line a Signal indicating that the call cannot be completed 75due to seizure of the trunk at the other end, and means at each end ofthe trunk and controlled by the calling subscriber for terminating themarking of said trunk as busy, said means being effective to terminatethe busy marking of said trunk at the end to which the calling line isconnected after the marking of said trunk as busy is terminated at theend to which the called line is connected.

5. In a telephone system, subscribers lines, a two-way trunk, meanscomprising a selector switch for connecting a calling line to saidtrunk, means comprising another selector switch for connecting saidtrunk to a called line, means responsive to seizure of said trunk formarking said trunk busy at both ends, means at each end of said trunkeiective upon simultaneous connection of calling lines to both ends ofsaid trunk for preventing the connection of said trunk to a called lineat either end, means at one end of the trunk for transmitting to acalling line a signal indicating that the call cannot be completed dueto seizure of the trunk at the other end, like signaling means at theother end of the trunk, and means at each end .of the trunk andcontrolled by the calling subscriber for terminating the marking of saidtrunk as busy, said means being effective to terminate the busy markingof said trunk at the end to which the calling line is connected afterthe marking of said trunk as busy is terminated at the end t'o which thecalled line is connected.

6. In a telephone system, subscribers lines, a two-way trunk arrangedfor composite signaling thereover, means comprising a selector `switchfor connecting a calling line to one end of said trunk, like means forconnecting a calling line to the other end of said trunk, meanscomprising a selector switch for connecting said one end of the trunk toa called line, like means for connecting the other end of the trunk to acalled line, means responsive to seizure of said trunk for marking itbusy at both ends, means responsive to release of the connection by thecalling subscriber for terminating the marking of said trunk as busy andfor causing the return to normal of the selector switches through whichthe trunk is connected to the calling and called lines, `and means fordelaying the return of the one of the last-mentioned switches whichconnects the calling line to the trunk until a suiicient interval oftime has elapsed for the return to normal of the other one of thelast-mentioned switches.

7. In a telephone system according to claim 6, means responsive toseizure of said trunk at the end connected to said called line fortemporarily interrupting the marking of the trunk as busy at the endconnected to said calling line if the lastmentioned seizure occurs priorto the release of the switch through which said `calling line isconnected to the trunk.

8. In a telephone system, subscribers lines, a two-way trunk, meanscomprising a selector switch for connecting a calling line to saidtrunk, means comprising another selector switch for connecting saidtrunk to a called line, means responsive to seizure of said trunk formarking said trunk busy at both ends, and means at each end of the trunkand controlled by the calling subscriber for terminating the marking ofsaid trunk as busy, said means comprising three slow-to-release relaysreleased in succession at the end of the trunk to which the calling lineis connected and comprising only two slow-to-release relays released insuccession at the end of the trunk to which the called line isconnected. 9. In a telephone system, subscribers lines, a two-way trunkarranged for composite signaling thereover, means comprising a selectorswitch for connecting a calling line to said trunk, means comprisinganother selector switch for connecting said trunk to a called line,means responsive to seizure of said trunk for marking said trunk busy atboth ends, means at each end of the trunk and controlled by the callingsubscriber for terminating the marking of said trunk as busy, said meansbeing effective to terminate the busy marking of said trunk at the endto which the calling line is connected after the marking of said trunkas busy is terminated at the end to which the called line is connected,and means effective upon simultaneous connection of calling lines toboth ends of said trunk for preventing the `connection of said trunk toa called line at either end and for transmitting a tone signal to eachof the calling subscribers.

10. In a telephone system, subscribers lines, a two-way trunk, meanscomprising a selector switch for connecting a calling line to saidtrunk, means comprising another selector switch for connecting saidtrunk to a called line, means responsive to seizure of said trunk formarking said trunk busy at both ends, means responsive to the release ofthe connection by the calling subscriber for terminating the marking ofsaid trunk as busy and for causing the return of said selector switchesto normal, means for delaying the return of the switch through which thecalling line is connected to said trunk until a suificient interval oftime has elapsed for said switch through which the trunk is connected tothe called line to have been restored to normal, and means responsive toseizure of said trunk at the end connected to said called line fortemporarily interrupting the marking of the trunk as busy at the endconnected to said calling line if the last-mentioned seizure occursprior to the release of the switch through which said calling line isconnected to the trunk.

11. In a telephone system, subscribers lines, a two-way trunk, meanscomprising a selector switch for connecting a calling line to one end ofsaid trunk, like means for connecting a calling line to the other end ofsaid trunk, means comprising a selector switch for connecting said oneend of the trunk to a called line, like means for connecting the otherend of the trunk to a called line, means responsive to seizure of saidtrunk for marking it busy at both ends, means responsive to release ofthe connection by the calling subscriber for terminating the marking ofsaid trunk as busy and for causing the return to normal of the selectorswitches through which the trunk is connected to the calling and calledlines, means for delaying the return of the one of the last-mentionedswitches which connects the calling line to the trunk until a suflicientinterval of time has elapsed for the return to normal of the other oneof the last-mentioned switches, and means responsive to seizure of saidtrunk at the end connected to said called line for temporarilyinterrupting the marking of the trunk as busy at the end connected tosaid calling line if the last-mentioned seizure occurs prior to therelease of the switch through which said calling line is connected tothe trunk.

HENRY HOVLAND.

